Recently, to realize an LCD or an organic EL display of a huge size and with high resolution, a high-speed and high-resolution image sensor, a three-dimensional IC and so on, research and development of fabricating high-performance semiconductor devices on either an insulating substrate such as a glass substrate or an insulating film has been carried on. Among other things, an LCD, including pixel portions and a driver on the same substrate, has been used as a monitor for not only personal computers (PCs) but also other general consumer electronic appliances. For example, more and more cathode-ray tubes (CRTs) have been replaced with LCDs as TV monitors and front projectors for viewing movies or enjoying games for amusement purposes have been purchased by an increasing number of general consumers nowadays, thus expanding the LCD market dramatically. Meanwhile, a so-called “system on panel” including a memory circuit and a logical circuit such as a clock generator that are integrated together on a glass substrate has also been researched and developed by a lot of engineers.
To present a high-resolution image, the amount of information to be written on pixels needs to be increased. And unless that huge amount of information is written in a short time, it is impossible to present the image, having an enormous amount of information to realize high-definition video, cannot be presented as moving pictures, for example. That is why a thin-film transistor (TFT) for use in a driver needs to operate very fast. And to realize such a high-speed operation, the TFT should be fabricated using a crystalline semiconductor film that has such a high degree of crystallinity as to realize sufficiently high field effect mobility.
Examples of known methods of forming a semiconductor film with good crystallinity on a glass substrate include a laser annealing process, in which an amorphous semiconductor film is irradiated with a laser beam such as excimer laser beam so as to be melted, solidified and crystallized instantaneously, and a solid-phase growth process, in which an amorphous semiconductor film is not heated to its melting point but thermally treated at around 600° C. to produce crystal growth. Besides these processes, a technique of forming a quality semiconductor film with highly aligned crystallographic orientations by conducting a heat treatment as a lower temperature and in a shorter time than conventional processes has also been developed. More specifically, a metallic element that promotes crystallization is added as a catalyst element to an amorphous semiconductor film and then the semiconductor film is subjected to a heat treatment. Even according to this technique, to further improve the crystallinity, a method of obtaining a crystalline semiconductor film of even higher quality by further irradiating the crystalline semiconductor film, formed by the heat treatment, with a laser beam is often adopted such that the film is partially melted, solidified and recrystallized and has its crystal defects reduced.
However, it is known that according to such a method of crystallizing an amorphous semiconductor film or recrystallizing a crystalline semiconductor film by irradiating the film with a laser beam and melting and solidifying the film, the surface of the resultant crystalline semiconductor film will get uneven. Such surface unevenness is produced as follows. Specifically, after the crystalline semiconductor film has been once melted by being irradiated with a laser beam, crystal nuclei are created and the film is sequentially solidified from those nuclei. In the meantime, as those molten portions and solidified portions have mutually different expansivities, crystal grain boundary portions to get solidified last tend to be raised and form a sort of a range of mountains. Or at a triplet point or a multi-crystal intersection where three or more crystals intersect with each other, a mountainous protrusion is formed. Such a portion that is raised like a range of mountains or like a mountain on the surface of the crystalline semiconductor film will be referred to herein as a “ridge”.
It is known that the ridge formed in this manner affects the characteristics of a TFT significantly. For example, since the top (or edge) of the ridge is sharp, the electric field is often concentrated there to generate leakage current, decrease the breakdown voltage of the gate insulating film, and eventually deteriorate the overall reliability of the device including hot carrier resistance. On top of that, if there is a ridge, the gate insulating film, deposited by a sputtering process or a CVD process, has decreased step coverage to make the electrical insulation insufficient. Furthermore, if the device is a top gate type thin-film transistor, the ridge will be located in the interface between the gate insulating film and the channel, and therefore, may sometimes affect the interface characteristic or decrease the field effect mobility. It is known that the field effect mobility of a TFT heavily depends on the degree of flatness of the interface between the TFT's active layer and the gate insulating film. That is to say, the flatter the interface, the higher the field effect mobility.
That is why various methods have been proposed to reduce those ridges (protrusions) on the surface of a crystalline semiconductor film that would affect the characteristics of a TFT.
For example, Patent Document No. 1 discloses a method of preventing those protrusions from being formed on the surface of a polycrystalline semiconductor film by depositing islands of an amorphous semiconductor material with a sloped edge and irradiating those islands with a laser beam within an oxidizing atmosphere. According to the method disclosed in Patent Document No. 1, a quality polycrystalline semiconductor film can be obtained. However, as the cooling rates are different between the edge portion and the body portion (i.e., the other portion) of the islands, crystallization advances from the edge of the islands of the amorphous semiconductor material, thus also forming a raised ridge on the edge. What is more, according to the method of Patent Document No. 1, the crystal grain size increases at the edge to make the grain sizes different between the edge and body portions. Or according to the method disclosed in Patent Document No. 5 (to be described later), the crystal grain size decreases at the edge to make the grain sizes different between the edge and body portions.
Each of Patent Documents No. 2, 3 and 4 discloses a method of reducing those ridges on the surface of a crystalline semiconductor film by performing the laser exposure process twice. More specifically, first, an amorphous semiconductor film is irradiated with a laser beam for the first time within an oxidizing atmosphere, thereby forming a polysilicon film. After that, in order to remove those ridges from the edges, the polysilicon film is irradiated with a laser beam for the second time within a vacuum or an inert gas.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 8-213637,        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2001-60551,        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2000-340503,        Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 2003-142402, and        Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 2000-101090        